1. Field of the Invention
The present invention relates to a method for annealing a semiconductor, in particular, to an annealing process for obtaining a polycrystal semiconductor used in a thin film device such as an insulated gate field effect transistor by laser irradiation.
2. Description of Prior Art
Thin films of polycrystalline silicon semiconductor for use in a tin film device such as a thin film insulated gate field effect transistor (abbreviated hereinafter as a TFT) have been fabricated heretofore by first depositing amorphous silicon films by plasma-assisted chemical vapor deposition (CVD) or thermal CVD processes, and then irradiating a laser beam thereto to crystallize the thus deposited amorphous silicon films.
The process of crystallizing an amorphous silicon film by irradiating a laser beam comprises, in general, first irradiating a low energy density laser beam to the amorphous silicon film to allow desorption of hydrogen having incorporated into the starting amorphous silicon film, and then irradiating a laser beam at an energy density well above the threshold energy density (a minimum energy density necessary to initiate melting of silicon).
A laser beam having a sufficiently low energy should be irradiated to the amorphous silicon film for the purpose of releasing the hydrogen being incorporated in the film because, if a beam of a high energy density corresponding to the threshold value or higher were to be irradiated, there occur two problems. One is a problem which involves abrupt evolution of a considerable amount of hydrogen from the surface of an amorphous silicon film upon irradiating the laser beam. Such a phenomenon greatly impairs the smoothness of the film surface; the resulting film cannot provide a favorable interface level when an insulator film is established on the surface of the thus crystallized silicon film, because a level develops at the interface between the silicon film, and the insulator film. The other problem is the hydrogens present in large amount in the amorphous silicon film; they not only evolve out of the surface upon irradiation of a high energy laser beam having an energy density not lower than the threshold value, but also move inside the melting silicon film with a large kinetic energy to impede the crystallization of the silicon itself.
Accordingly, a conventional laser annealing processes involves a so-called pre-laser annealing step which comprises irradiating a low energy density laser beam to sufficiently drive out hydrogen atoms having incorporated inside the film, followed by the irradiation of a laser beam having a satisfactorily high energy density to effect crystallization of the film. In this manner, the influence of the hydrogen inside the film on the film crystallization can be eliminated.
The conventional laser annealing processes, however, suffer disadvantages as follows.
Firstly, the laser annealing process should be conducted in two steps. Such a process is not suitable for processing large-area substrates. Moreover, it suffers poor efficiency.
Secondly, the most generally used excimer lasers which are operated in a pulsed mode are not suitable for completely driving hydrogen out of the film; the duration of the laser irradiation per pulse is too short.
Furthermore, any laser apparatus for use in the laser annealing inevitably suffers a non-uniform laser beam output and a fluctuation in power output. Those attributes make the hydrogen distribution non-uniform inside the film upon driving hydrogen atoms out of the film. Such a film having a non-uniform hydrogen distribution therein results in a crystallized film consisting of crystal grains of non-uniform grain diameter.
The present invention relates to a laser annealing process having overcome the aforementioned problems.
More specifically, the present invention provides a method for annealing a semiconductor comprising the steps of:
thermally annealing an amorphous semiconductor in vacuum or inactive atmosphere at a temperature not higher than a crystallization temperature of said amorphous semiconductor; and
irradiating said amorphous semiconductor with a laser light in vacuum or inactive atmosphere after said thermally annealing step to crystallize said amorphous semiconductor.
The laser to be used in the process in general is an excimer laser, but it should be noted that the construction of the present invention is by no means restricted to the use thereof, and hence, any type of laser can be employed in the process.
The generally used amorphous semiconductor, but not limiting, is a silicon semiconductor. In the description of the present invention hereinafter, however, a silicon semiconductor is used for purpose of explanation.
The thermal annealing of the amorphous semiconductor in vacuum or in an inactive gas atmosphere at a temperature not higher than the crystallization temperature of said amorphous semiconductor is conducted for the purpose of driving hydrogen out of the amorphous semiconductor. If this step of thermal annealing were to be conducted at a temperature not lower than the crystallization temperature of the amorphous semiconductor, crystallization would occur on the semiconductor, thereby making the subsequent crystallization by laser irradiation insufficient. Accordingly, it is an important point that the thermal annealing is conducted at a temperature not higher than the crystallization temperature of the semiconductor.
The thermal annealing step should be effected in vacuum or in an inactive gas atmosphere to avoid formation of an undesirable thin film, such as an oxide film, on the surface of the amorphous semiconductor.
Hydrogen can be uniformly and thoroughly driven out of the film by annealing the amorphous semiconductor at a temperature not higher than the crystallization temperature. The semiconductor films thus obtained have improved uniformity for both the intra-planar distribution of crystallinity and the size of the constituting crystal grains. Such semiconductor films enable fabrication of polycrystalline silicon (abbreviated sometimes as xe2x80x9cp-Sixe2x80x9d, hereinafter) TFTs having uniform characteristics over a large-area substrate.
The crystallization of the amorphous semiconductor in vacuum or in an inactive gas atmosphere by irradiating a laser beam thereto is conducted to prevent the dangling bonds, which have once formed upon driving hydrogen out of the amorphous semiconductor, from bonding with oxygen and hydrogen and nitrogen present in the active gas, i.e., air.
The present invention is characterized in one aspect that a large amount of dangling bonds are produced in the amorphous semiconductor to accelerate crystallization of the film. This is based on the fact obtained experimentally by the present inventors, which is described below. It has been found that the crystallinity of an amorphous silicon film having subjected to a thorough driving out of hydrogen remarkably improves by irradiating an excimer laser light (a KrF laser emitting light at wavelength 248 nm) to the film.
An amorphous silicon film in general contains a large amount of hydrogen to neutralize the dangling bonds within the amorphous silicon film. The present inventors, however, realized the important role which the dangling bonds play at the crystallization of the film from its amorphous molten state, and therefore intentionally allowed the dangling bonds to form in the amorphous state to enhance instantaneous crystallization from the molten state. In the course of the crystallization taking advantage of the thus formed dangling bonds, it is very important to irradiate the laser beam in vacuum or in an inactive gas atmosphere, as mentioned earlier, because the exposure of the surface of the thus obtained semiconductor film to air causes bonding (neutralization) of the dangling bonds with oxygen, etc., to form an oxide film and the like on the surface of the film.
The annealing according to the process of the present invention should be conducted at a temperature not higher than the crystallization temperature of the amorphous semiconductor. The crystallization temperature as referred herein signifies the temperature at which the amorphous semiconductor initiates crystallization by thermal annealing. The thermal annealing at a temperature not higher than the crystallization temperature according to the process of the present invention is conducted on the basis of an experimentally derived fact that the improvement of crystallinity is hardly observed by irradiating a laser beam to a once crystallized film, and that those crystallized films are considerably low in crystallinity as compared with the films having crystallized by irradiating a laser beam to films still in their amorphous state.
It can be seen, accordingly, that it is of great importance to drive the hydrogen atoms out of the amorphous semiconductor film at a temperature not higher than the crystallization temperature of the amorphous semiconductor film. However, hydrogen atoms are preferably driven out of the amorphous semiconductor by thermal annealing at a temperature as high as possible, provided that the film does not initiate crystallization; it is of grave importance in the process according to the present invention to form as many dangling bonds as possible in the film while thoroughly driving hydrogen out of the film.
The thermal annealing of the film to drive hydrogen out of the film is characterized by that, unlike the conventional processes which use laser beams at a low energy density, it enables a uniform and thorough elimination of hydrogen from the amorphous semiconductor film.
The process according to the present invention therefore is of great advantage concerning that it realizes a polycrystalline semiconductor film composed of large and uniform crystal grains.